The present invention relates to broadcast transmitters, and more particularly, to such transmitters that use digital techniques to achieve amplitude modulation.
The use of digital techniques to synthesize an AM (amplitude modulated) RF (radio frequency) waveform is known from U.S. patent application Ser. No. 217,795, filed Dec. 18, 1980 in the names of B. J. Lechner and F. J. Marlowe and assigned to the assignee of the present application. In said application, an audio signal is digitized (periodically sampled and then quantized into a plurality of parallel bits), and each of the resulting plurality of parallel bits is used to control one of a like plurality of class D RF bridge amplifiers. The RF signals from the bridges are applied to primary windings of respective RF transformers having series-coupled secondary windings. Thus the transformed RF current from all of the bridges flows through all of the secondary windings, thereby requiring a relatively large diameter wire for all secondary windings as well as transformer cores sufficiently large to carry the large diameter wire.
Further, at least one embodiment shown in the aforementioned application requires a binary progression of effective turns ratios for the respective RF transformers, e.g. 1:1, 1:2, 1:4, 1:8, . . . 1:128, for eight-bit digitization. The higher of these effective turns ratios may be difficult to achieve due to factors such as winding machine turns counter errors. Further, at high turns ratios, the desired high coupling between primary and secondary windings for maximum RF efficiency may also be difficult to achieve, since it may be difficult to couple e.g. one primary turn to 128 secondary turns.
It is therefore desirable to provide a digital AM transmitter that has RF transformers that have easy-to-implement turns ratios with tight coupling and in which at least some of the transformers may be made with small-diameter secondary wire and small-diameter cores. Further, it is desirable to have at least some identical transformers for purposes of economy.